Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Emission Tuning of Heteroleptic Arylborane−Ruthenium(II) Complexes by Ancillary Ligands: Observation of Strickler−Berg-Type Relation Atsushi Nakagawa,† Akitaka Ito,§ Eri Sakuda,∥ Sho Fujii,†,‡ and Noboru Kitamura*,†,‡
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Department of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering and ‡Department of Chemistry, Faculty of Science, Hokkaido University, Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810, Japan § Graduate School of Engineering/School of Environmental Science and Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan ∥ Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Bunkyo-machi, Nagasaki 852-8521, Japan S Supporting Information *
ABSTRACT: Novel heteroleptic arylborane−ruthenium(II) complexes having a series of ancillary ligands L′ ([Ru(B2bpy)L′2]2+) in CH3CN showed low-energy/intense metal-to-ligand charge transfer (MLCT)-type absorption and intense/long-lived emission compared to the reference complexes. The spectroscopic and photophysical properties of [Ru(B2bpy)L′2]2+ were shown to be manipulated synthetically by the electron-donating ability of the ancillary ligand(s). The intense and long-lived emission observed for [Ru(B2bpy)L′2]2+ in CH3CN at 298 K is responsible for the accelerated radiative and decelerated nonradiative decay processes, which are controllable through the electronic structures of the ancillary ligand(s). On the basis of the present systematic study, furthermore, we succeeded in demonstrating the Strickler−Berg-type relation between the molar absorption coefficients of the MLCT bands and the radiative rate constants of the complexes.
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INTRODUCTION The intense visible absorption and emission from tris-diimine ruthenium(II) ([RuL3]2+) complexes have been hitherto applied to sensing,1−5 light-emitting materials,6,7 artificial photosynthesis,8−11 and so forth.12−15 In particular, the complexes are often utilized as photosensitizers owing to their intense metal-to-ligand charge transfer (MLCT) absorption (molar absorption coefficient (ε) ≈ 104 M−1 cm−1 at ∼450 nm), long-lived excited states (∼1 μs), and high stabilities in the multiple redox states. Although the complexes show phosphorescence from the triplet MLCT (3MLCT) excited states even in solution at room temperature, the emission quantum yields (Φem) of most of the complexes are smaller than 0.1;16 therefore, development of highly emissive [RuL3]2+ complexes is of primary importance. Since the Φem value of a complex is determined primarily by the radiative (kr) and nonradiative decay rate constants (knr) through the relation Φem = kr/(kr + knr), the simplest approach to enhance Φem is an increase in kr and a decrease in knr. However, the knr value of [RuL3]2+ is typically 10 times larger than kr owing to a large contribution of nonradiative decay of the 3MLCT excited state via the nonemissive triplet dd excited (3dd*) state,17 and such large knr impedes a development of a highly emissive [RuL3]2+ complex. Furthermore, a control of kr © XXXX American Chemical Society
has been rarely explored due to its experimental difficulties. Under such circumstances, a systematic study on the spectroscopic and photophysical properties of [RuL3]2+ complexes is absolutely required to obtain a new synthetic strategy toward an enhancement of the emission efficiency of the complex. We have focused on chemical decorations of an MLCT-type transition metal complex by a triarylborane unit(s).18−25 Several research groups have also reported characteristic spectroscopic and photophysical properties of aryiboranedecorated platinum(II),26−38 iridium(III),39−47 ruthenium(II),48,49 zinc(II),50,51 copper(I),26,27 rhenium(I),34,52 and gold(I) complexes.34 In the transition metal complex possessing a triarylborane-appended ligand(s), the intramolecular CT transition from the π-orbital of the aryl group (π(aryl)) to the vacant p-orbital on the boron atom (p(B)), π(aryl)−p(B) CT, in the triarylborane moiety synergistically participates in the MLCT transition in the metal-complex moiety, and as a result, the complex shows intriguing spectroscopic and excited-state properties.18−25 We recently reported the characteristic emission properties of a homoleptic Received: April 17, 2018
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DOI: 10.1021/acs.inorgchem.8b01058 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry arylborane−ruthenium(II) complex ([Ru(B2bpy)3]2+), in which six (dimesityl)boryldurylethynyl (DBDE) groups were introduced to [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) at the 4and 4′-positions of the three bpy ligands: B2bpy = 4,4′(DBDE)2-2,2′-bipyridine.22 In CH3CN at 298 K, [Ru(B2bpy)3]2+ shows extremely intense and long-lived emission (Φem = 0.43 and τem = 1.7 μs). The results demonstrate that the DBDE group is a fascinating functional group for a development of highly emissive transition metal complexes based on the synergistic MLCT/π(aryl)−p(B) CT interactions. We have demonstrated that such characteristic emission of [Ru(B2bpy)3]2+ is derived from the large kr (2.5 × 105 s−1) and small knr values (3.4 × 105 s−1) of the complex compared to the relevant values of [Ru(bpy)3]2+ (kr = 1.1 × 105 s−1 and knr = 1.0 × 106 s−1). Although the small knr value of [Ru(B2bpy)3]2+ has been explained by the suppression of nonradiative decay of the emissive excited state via the 3dd* state, the origin of the large kr value of the complex has not been sufficiently elucidated. A useful insight into a development of highly emissive ruthenium(II) complexes has been obtained from the emission spectral features of heteroleptic arylborane−ruthenium(II) complexes: [Ru(B2bpy)n(bpy)3−n]2+ (n = 1−3).22 Since the emission spectra of [Ru(B2bpy)n(bpy)3−n]2+ are almost identical to each other, the excited electron in the emissive triplet excited state is predicted to be localized primarily on one arylboraneappended ligand in the complex. This indicates that the two ligands other than the arylborane-appended ligand in a heteroleptic tris-diimine ruthenium(II) complex can be utilized as ancillary ligands to control the electronic structures of an arylborane−ruthenium(II) complex in both ground and excited states. In this study, we have synthesized novel heteroleptic arylborane−ruthenium(II) complexes having a series of bpyor 1,10-phenanthlorine (phen)-based ancillary ligands L′, [Ru(B2bpy)L′2]2+: 1Ph (L′ = 4,4′-diphenyl-bpy (dpbpy)), 1Me (L′ = 4,4′-dimethyl-bpy (dmbpy)), 2Ph (L′ = 4,7diphenyl-phen (dpphen)), 2Me (L′ = 4,7-dimethyl-phen (dmphen)), and 2H (L′ = phen); see Chart 1 for the structures. The spectroscopic and photophysical properties of the complexes are then compared with those of [Ru(B2bpy)(bpy)2]2+ (1H) and the relevant reference complexes (i.e., [Ru(bpy)3]2+ and [Ru(bpy)(phen)2]2+), and we discuss the experimental results in terms of the ancillary-ligand effects. The particular emphasis of the present study is the observation of the Strickler−Berg-type relation between the ε values of the MLCT bands and the kr/Φem values of the complexes, whose relation could be utilized as the basic strategy toward development of bright phosphorescent transition metal complexes.
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Chart 1. Chemical Structures and Abbreviations of [Ru(B2bpy)L′2]2+
EXPERIMENTAL SECTION
Materials. 1H(PF6)2 and [Ru(bpy)3](PF6)2 are essentially the same with those reported in the earlier literature.22 [Ru(bpy)(phen)2](PF6)2 was prepared as described in the Supporting Information. The synthetic routes to [Ru(B2bpy)L′2]2+ are shown in Scheme 1, and as a typical example, the synthetic procedures of 1Ph are described below. Other complexes were prepared by the analogous procedures with those of 1Ph as reported in detail in Supporting Information. [Ru(COD)Cl2]n (COD = 1,5-cyclooctadiene),53 (ethynylduryl)dimesitylborane (EDDB),21 and 4,4′-diphenyl-2,2′-bipyridine (dpbpy)54 used for the syntheses of [Ru(B2bpy)L′2]2+ were prepared according to the literatures. All of other chemicals for the synthetic
experiments purchased from Wako Pure Chemical Ind., Ltd., Kanto Chemical Co. Inc., or Tokyo Chemical Ind. Co., Ltd. were used as supplied. Column chromatography was carried out by using the materials and equipment reported previously.22 1H NMR and electrospray ionization (ESI) mass spectroscopies were conducted by using a JEOL JME-EX270 FT-NMR system (270 MHz) and a Waters micromass ZQ spectrometer, respectively. The chemical shifts of the 1H NMR spectra in CD3CN were given in ppm, with tetramethylsilane being an internal standard (0.00 ppm). Elemental analyses were conducted in the Instrumental Analysis Division, Equipment Management Center, Creative Research Institution, Hokkaido University. B
DOI: 10.1021/acs.inorgchem.8b01058 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. Synthetic Routes for [Ru(B2bpy)L′2](PF6)2 (−R = −C6H5, −CH3, −H)a
a
(i) L′, o-dichlorobenzene, 160°C, 2 h; (ii) 4,4′-Br2-bpy, ethanol/water, 80°C, 5 h; (iii) EDDB, Pd(PPh3)2Cl2, CuI, NEt3, CH3CN/THF, 50°C, 4 h.
Synthesis of cis-Dichloridobis(4,4′-diphenyl-2,2′bipyridine)ruthenium(II) (cis-[Ru(dpbpy)2Cl2]). The synthesis was performed by the reported procedures with some modifications.55 A suspension of [Ru(COD)Cl2]n (70 mg, 0.25 mmol on the basis of the ruthenium atom) and dpbpy (150 mg, 0.49 mmol) in odichlorobenzene (6 mL) was heated at reflux temperature (160 °C) for 2 h. After cooling the suspension, the reaction mixture was added into diethyl ether (40 mL) and stirred for 30 min. The resulting precipitates were collected by filtration, affording cis-[Ru(dpbpy)2Cl2] as black powders (170 mg, 88%). This compound was used in the following step without any purification and identification. Synthesis of (4,4′-Dibromo-2,2′-bipyridine)bis(4,4′-diphenyl-2,2′-bipyridine)ruthenium(II) Bis(hexafluorophosphate) ([Ru(4,4′-Br2-bpy)(dpbpy)2](PF6)2). The synthesis was performed by the reported procedures with some modifications.56 A suspension of cis-[Ru(dpbpy)2Cl2] (60 mg, 0.076 mmol) and 4,4′-Br2-bpy (71 mg, 0.23 mmol) in 70 mL of an ethanol/water mixture (1/1, v/v) was heated at 80 °C for 5 h. After cooling the mixture to room temperature, ethanol was removed under reduced pressure. The insoluble residues were removed by filtration, and an excess amount of an NH4PF6 (aqueous) solution was added to the filtrate. The resulting precipitates were collected and purified by column chromatography (Al2O3, CH3CN/CHCl3 (1/1, v/v)). The crude product was dissolved in a minimum amount of acetone, and an excess amount of n-hexane was then added dropwise to the solution, giving [Ru(4,4′-Br2-bpy)(dpbpy)2](PF6)2 as red powder (55 mg, 55%). 1H NMR (270 MHz, CD3CN) δ/ppm 8.93 (4H, s, 3,3′-Ar−H of dpbpy), 8.80 (2H, d, J = 2.2 Hz, 3,3′-Ar−H of 4,4′-Br2-bpy), 7.98− 7.89 (8H, m, 5,5′,6,6′-Ar−H of 4,4′-Br2-bpy and 6,6′-Ar−H of dpbpy), 7.85 (4H, d, J = 6.0 Hz, 5,5′-Ar−H of dpbpy), 7.75−7.55 (20H, m, Ar−H of phenyl); MS (ESI) m/z 516.1 (calcd for [M − 2PF6]2+ (C54H38N6Br2Ru): 515.9). Synthesis of [4,4′-Bis{(dimesitylboryl)durylethynyl}-2,2′bipyridine]bis(4,4′-diphenyl-2,2′-bipyridine)ruthenium(II) Bis(hexafluorophosphate) (1Ph(PF6)2). An oven-dried Schlenk tube was evacuated and filled subsequently with an Ar gas. [Ru(4,4′-Br2bpy)(dpbpy)2](PF6)2 (50 mg, 0.038 mmol), CuI (2.9 mg, 0.011 mmol), and Pd(PPh3)2Cl2 (1.9 mg, 0.0027 mmol) were added to the tube; then, the tube was evacuated and filled with an Ar gas again. An Ar gas purged CH3CN/triethylamine mixture (1.2 mL/0.50 mL) was added to the reaction tube, and the resulting suspension was stirred at room temperature for 20 min. A tetrahydrofuran solution (3.5 mL) of EDDB (45 mg, 0.11 mmol) was then added dropwise to the reaction mixture. The mixture was stirred at 50 °C for 4 h under Ar gas atmosphere and then allowed to cool to room temperature. The insoluble residues were removed by filtration through Celite (No. 500), and the filtrate was evaporated to dryness. The crude product was dissolved in a minimum amount of acetone, and the solution was added dropwise to an excess amount of n-hexane, giving red precipitate. After the reprecipitation procedure was carried out
three times, purification by column chromatography (LH-20, CH3CN/ethanol, (1/1, v/v)) followed by preparative HPLC (chloroform) afforded 1Ph(PF6)2 as red powder (30 mg, 50%). 1H NMR (270 MHz, CD3CN, TMS) δ/ppm 8.96 (4H, s, 3,3′-Ar−H of dpbpy), 8.74 (2H, s, 3,3′-Ar−H of B2bpy), 7.95−7.84 (14H, m, 6,6′Ar−H of dpbpy, 6,6′-Ar−H of B2bpy and Ar−H of phenyl), 7.78− 7.43 (18H, m, 5,5′-Ar−H of dpbpy, 5,5′-Ar−H of B2bpy and Ar−H of phenyl), 6.80 (8H, s, Ar−H of mesityl group), 2.44 (12H, s, CH3, mpositions of duryl groups), 2.24 (12H, s, CH3, o-positions of duryl groups), 2.02 (12H, s, CH3, p-positions of mes groups), 1.94 (24H, s, CH3, o-positions of mesityl groups). MS (ESI) m/z 841.2 (calcd for [M − 2PF6]2+ (C114H106N6B2Ru): 841.4). Elemental analysis calcd (%) for C114H106N6B2RuP2F12·2.5CHCl3: C 61.34, H 4.79, N 3.68. Found: C 61.57, H 4.77, N 3.65. The elemental analysis indicated that the crystals of 1Ph involved CHCl3 molecules used for preparative HPLC as an eluent. Electrochemical Measurements. Cyclic and differential pulse voltammetries of the complexes (∼5 × 10−3 M) in N,Ndimethylformamide (DMF, for nonaqueous titrimetry, Kanto Chemical Co., Inc.) were conducted by using a BAS ALS-701D electrochemical analyzer with a three-electrodes system using glassy carbon working, Pt auxiliary, and Ag/Ag+ reference electrodes at 25 ± 2 °C. A small amount of ferrocene (Fc, used as supplied from Wako Pure Chemical Ind. Ltd.) was added to a sample solution as an internal standard for the redox potentials. Tetra-n-butylammonium hexafluorophosphate (TBAPF6, 0.1 M) was recrystallized from ethanol three times and used as a supporting electrolyte. Sample solutions were deaerated by purging an Ar gas stream over 20 min prior to measurements. The potential sweep rate was 100 mV/s in cyclic voltammetry, and the differential pulse voltammetry was conducted with 50-mV height and 60 ms width pulses stepped by 4 mV with 2.0 s interval between the two pulses. Spectroscopic and Photophysical Measurements. The absorption spectra of the complexes in spectroscopic-grade CH3CN (Wako Pure Chemical Ind. Ltd.) were measured by using a Hitachi U-3900H spectrophotometer. The corrected emission spectra of the complexes in Luminasol CH3CN (Dojindo Molecular Technologies, Inc.) were measured by using a Hamamatsu PMA-11 multichannel photodetector (excitation wavelength = 355 nm). For emission spectroscopy, the absorbance of a sample solution was set to 2Me (∼56%, −5.90 eV) > 1Ph (∼52%, −5.93 eV) > 2Ph (∼41%, −5.94 eV) > 1H (∼45%, −5.97 eV) > 2H (∼36%, −5.97 eV). The sequence of the HOMO energy mentioned above agrees with that of Eox. Since HOMO of [Ru(B2bpy)L′2]2+ is characterized by the contributions from both d-orbital of the ruthenium(II) atom and π-orbital of the B2bpy ligand, destabilization of the dorbital energy by electron donation from the ancillary ligands is expected to increase the HOMO energy and the contribution of the ruthenium atom to HOMO. In contrast, LUMO of [Ru(B2bpy)L′2]2+ is localized on the B2bpy ligand, irrespective of L′, and the LUMO distributions and energies (−2.73 to −2.77 eV) of the complexes are comparable with one another. These results indicate that LUMO of [Ru(B2bpy)L′2]2+ is insensitive to L′ similar to the results on Ered1. These redox potentials and MOs of the complexes suggest that the spectroscopic and photophysical properties of [Ru(B2bpy)L′2]2+ are characterized by the electronic structures of the ancillary and main ligand (i.e., B2bpy).
Figure 1. Absorption spectra of the complexes (1Ph, 1Me, 1H, and [Ru(bpy)3]2+ (a) and 2Ph, 2Me, 2H, and [Ru(bpy)(phen)2]2+ (b)) in CH3CN at 298 K. The data on 1H and [Ru(bpy)3]2+ were taken from ref 22.
Absorption Spectra. Figure 1 shows the absorption spectra of the complexes in CH3CN at 298 K. The absorption maximum wavelengths (λabs) and corresponding molar absorption coefficients (ε) are summarized in Table 3. All of the complexes show the intense absorption bands in
